1. Field of the Invention
This invention relates to laser markable laminates and documents, especially security documents.
2. Description of the Related Art
Security cards are widely used for various applications such as identification purposes (ID cards) and financial transfers (credit cards). Such cards typically consist of a laminated structure consisting of various papers or plastic laminates and layers wherein some of them may carry alphanumeric data and a picture of the card holder. So called ‘smart cards’ can also store digital information by including an electronic chip in the card body. A principal objective of such articles and security cards is that they cannot be easily modified or reproduced in such a way that the modification or reproduction is difficult to distinguish from the original.
Two techniques frequently used for preparing security documents are laser marking and laser engraving. In literature, laser engraving is often incorrectly used for laser marking. In laser marking, a colour change is observed by local heating of material, while in laser engraving material is removed by laser ablation.
Well known in the field of laser markable security documents is the use of laser markable polymeric supports. Laser marking produces a colour change from white to black in a laser markable support through carbonization of the polymer, usually polycarbonate as disclosed in e.g. EP-A 2181858 (AGFA GEVAERT).
During the past last years, there is an increased interest of using laser markable layers. The advantage of using a laser markable layer coated on a support instead of a laser markable support, is that a support can be used which has better physical properties than the laser markable supports, such as for example a higher flexibility than a polycarbonate support as disclosed in e.g. EP-A 2567825 (AGFA GEVAERT).
Laser markable layers typically contain colour forming compounds (also called “leuco-dyes”) which can change from essentially colourless or pale-coloured to coloured when irradiated with UV light, IR light and/or heated. Different classes of leuco dyes are well known and widely used in conventional pressure-sensitive, photosensitive or thermally-sensitive recording materials (“Chemistry and Applications of Leuco Dyes”, Ramaiah Muthyala, Plenum Press, 1997).
A disadvantage of a material, for example a colour laser markable laminate including such a colour laser markable layer, may be their limited daylight stability. For that reason, so-called daylight stabilizers may be added to the colour laser markable layer.
Daylight stabilizers can be classified in 5 classes: UV-absorbers (such as for example hydroxybenzophenones, triarylimidazoles, 2-hydroxyphenyl-s-triazines and hydroyphenyl-benzotriazoles), quenchers (such as for example nickel phenolates), hydroxyperoxide decomposers (such as for example dialkyldithiocarbamates, dialkyldithiophosphates and thiobisphenolates), antioxidants (such as for example phenolic antioxidants, phosphites, phosphonites and sulfur compounds) and hindered amine light stabilizers (HALS). More information on daylight stabilizers can be found in “Plastics Additives Handbook, 5th Edition”, Hans Zweifel, HANSER, 2001).
EP0941866 discloses a thermally imageable element having an improved light stability comprising a leuco-dye, optionally an IR-absorbing dye and optionally an UV-absorber such as polyhydroxybenzophenones, triarylimidazoles or hydroyphenylbenzotriazoles.
JP06328843 (FUJI) discloses a laser imageable recording material with leuco-dyes and an UV-absorber for improved lightfastness.
WO2008122504 (CIBA) describes the use of leuco-dyes in combination with UV-absorbers and/or HALS for an improved light stability.
EP2639074 (AGFA) discloses a colour laser markable laminate including at least a transparent polymeric support and a colour forming layer comprising a leuco-dye; an infrared dye; and a polymeric binder comprising vinyl acetate and at least 85 wt % of vinyl chloride based on the total weight of the binder; wherein the colour laser markable laminate includes a specific phenol stabilizer sterically hindered by a ring containing three nitrogen atoms.
However, the addition of daylight stabilizers may negatively impact the performance of the laminate, such as giving significantly less colour formation upon laser marking (loss of sensitivity). Such a lower sensitivity requires longer exposure time of the laser.
For laser markable layers, the use of acid-sensitive leuco-dyes is preferred since it includes both advantages of the blister formation security feature and the enhanced shelf-life stability, such as disclosed in e.g. EP-A 2463110 (AGFA). However, acid-sensitive leuco-dyes for laser markable layers are typically used in combination with acid-generating compounds such as photoacid-generators (PAG). The use of such acid-generating compounds has the disadvantage that they may not be daylight stable.
WO2007063339 (DATALASE) discloses a laser markable composition comprising a dye responsive to the presence of hydrogen ions, a compound that generates an acid on irradiation and optionally an anti-oxidant to improve the daylight stability.
U.S. Pat. No. 5,858,583 (DUPONT) discloses a thermally imageable element comprising a HABI compound, a leuco dye, an acid-generating compound and a near IR absorbing dye. The daylight stability can be improved by the addition of UV absorbers and/or antioxidants.
A further requirement of colour laser markable laminates for security cards is an excellent thermal stability. Lamination is usually performed at elevated temperatures in order to achieve a good adhesion between the different layers. Laser markable layers comprising leuco-dyes and acid generators may have a limited thermal stability which results in background stain of the laser markable material after lamination at elevated temperatures.